In recent years, electric power steering devices employing an electric motor in a steering assist mechanism are commonly employed as steering devices for automobiles. Such a conventional steering device typically transmits a rotational torque generated by the electric motor to a steering column as supplemental torque by way of a worm gear.
However, because transmission efficiency of worm gears is in such a low range as 60% to 80% in general, a larger output torque from the electric motor is needed in order to maintain a constant reduction ratio, thereby leading to a larger external diameter of the electric motor. Therefore, such electric power steering devices adopting a reduction gear unit with a worm gear therein have a problem that it is difficult to reduce overall dimensions of the electric power steering devices. In order to solve such a problem, the reduction gear unit having an output shaft of an electric motor mounted in parallel to a steering column and spur gears or helical gears as a reduction mechanism have been developed in these days.
Because of relatively high transmission efficiency, which is about 95%, of spur gears or helical gears, output torque generated by an electric motor can be reduced correspondingly. As a result, excessive enlargement of an external dimension of the electric motor can be avoided, so that electric power steering devices can be configured to be compact as a whole.
However, an attempt to obtain a desired reduction ratio for a reduction mechanism adopting a spur gear by a single-stage configuration, in which a gear mounted on an output shaft of an electric motor is meshed simply with a gear mounted on a steering column, makes no contribution to an effort to overcome the difficulty in configuring an electric power steering device into a compact shape as a whole due to an increase in the pitch circle of the driven gear, i.e., the gear on the steering column.
On the other hand, in case of a reduction gear unit employing a multi-stage configuration by interposing, for example, some intermediate gears rather than the simple, single-stage configuration adopting spur gears, new problems arise such as a decrease in the comfortable feeling of a steering operation due to an enlargement of the backlash, an increase in costs caused by a magnified structural complexity of the reduction gear unit, and the like.
In order to solve such problems, Patent Document 1, for example, discloses a steering device which can enable to configure a steering column assembly having an electric motor and a reduction gear unit into a compact shape as a whole through disposing the reduction gear unit comprising a pair of spur gears or helical gears set to have a high reduction ratio in a housing and mounting the electric motor proximately to the housing.
Also, Patent Document 2 discloses a reduction gear unit in which a housing enclosing gears is filled with an oil and fat lubricant in order to reduce vibrations and the like caused by gear rattle when a worm gear has been used.
Patent Document 1 Japanese Patent Application Laid-Open No. 11-124045
Patent Document 2 Japanese Patent Application Laid-Open No. 2002-147575